U.S. patent number 5,058,556 [Application Number 07/638,844] was granted by the patent office on 1991-10-22 for device for determining activated condition of an oxygen sensor.
This patent grant is currently assigned to Toyota Jidosha Kabushiki Kaisha. Invention is credited to Takao Fukuma, Toshio Takaoka, Keisuke Tsukamoto, Hirofumi Yamasaki.
United States Patent |
5,058,556 |
Fukuma , et al. |
October 22, 1991 |
Device for determining activated condition of an oxygen sensor
Abstract
A device for determining an activated condition of the oxygen
sensor of a lean sensor for an electric-controlled fuel injection
internal combustion engine. When the engine is under a fuel cut
operation, the output level from the oxygen sensor is compared with
a first predetermined threshold level or with a second
predetermined threshold level which is lower than the first
threshold level. A determination of the activated condition of the
oxygen sensor is obtained when the output level from the oxygen
once exceeds the higher first level, or when the output level from
the oxygen sensor consecutively exceeds the higher first level
twice, whereby a positive and quick determination of the activated
condition of the oxygen sensor can be obtained.
Inventors: |
Fukuma; Takao (Toyota,
JP), Tsukamoto; Keisuke (Toyota, JP),
Takaoka; Toshio (Toyota, JP), Yamasaki; Hirofumi
(Toyota, JP) |
Assignee: |
Toyota Jidosha Kabushiki Kaisha
(Toyota, JP)
|
Family
ID: |
11854288 |
Appl.
No.: |
07/638,844 |
Filed: |
January 8, 1991 |
Foreign Application Priority Data
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Jan 23, 1990 [JP] |
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2-14193 |
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Current U.S.
Class: |
123/682;
123/688 |
Current CPC
Class: |
G01N
27/4065 (20130101); F02D 41/148 (20130101) |
Current International
Class: |
F02D
41/14 (20060101); G01N 27/406 (20060101); F02D
041/14 () |
Field of
Search: |
;123/325,326,440,489,479
;204/406,425,426,427,428 |
References Cited
[Referenced By]
U.S. Patent Documents
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|
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4186691 |
February 1980 |
Takase et al. |
4648370 |
March 1987 |
Kobayashi et al. |
4707241 |
November 1987 |
Nakagawa et al. |
4723521 |
February 1988 |
Mieno et al. |
4724814 |
February 1988 |
Mieno et al. |
4759328 |
July 1988 |
Blumel et al. |
4958612 |
September 1990 |
Kato et al. |
4964271 |
October 1990 |
Sawada et al. |
4981122 |
January 1991 |
Osawa et al. |
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Foreign Patent Documents
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59-46350 |
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Mar 1984 |
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JP |
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60-212650 |
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Oct 1985 |
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JP |
|
Primary Examiner: Wolfe; Willis R.
Attorney, Agent or Firm: Kenyon & Kenyon
Claims
What is claimed:
1. A device for determining an activated condition of an oxygen
sensor for an internal combustion engine when a fuel cut operation
is carried out under a predetermined condition of the engine,
comprising:
means for detecting a state in which the fuel cut operation is
carried out;
first comparing means for comparing a detected level of the sensor
means with a predetermined first value when the engine is under a
fuel cut condition;
second comparing means for comparing a detected level of the sensor
means with a predetermined second value which is lower than said
first value when the engine is under a fuel cut condition;
first determining means for determining an activated condition of
the oxygen sensor when the first comparing means determines that
the detected level once exceeds the first value, and;
second determination means for determining an activated condition
of the oxygen sensor when the second comparing means determines
that the detected level consecutively exceeds the second value at
least twice.
2. An internal combustion engine comprising:
an engine body;
an intake line connected to the engine body for an introduction of
intake air into the engine body;
mean for introducing an amount of fuel into the intake line for
forming an air-fuel mixture;
an exhaust line connected to the engine body for a removal of a
resultant exhaust gas from the engine body;
means arranged in the exhaust line and responsive to the oxygen
density of the oxygen in the exhaust gas for producing an electric
signal having a level which indicates the air-fuel ratio of the
combustible air-fuel mixture introduced into the engine;
means, responsive to a signal from the sensor means, for
controlling the amount of the supplied fuel to thereby control the
air-fuel ratio to a predetermined value;
means, responsive to a predetermined condition of the engine, for
cutting the supply of fuel to the engine;
means for cutting the supply of fuel to the engine from the fuel
supply mean;
first comparing means for comparing the detected level of the
sensor means with a predetermined first value when the engine is
under a fuel cut condition;
second comparing means for comparing the detected level of the
sensor means with a predetermined second value which is lower than
said first value when the engine is under a fuel cut condition;
first determining means for determining an activated condition of
the oxygen sensor when the first comparing means determines that
the detected level once exceeds the first value;
second determining means for determining an activated condition of
the oxygen sensor when the second comparing means determines that
the detected level consecutively exceeds the second value at least
twice, and;
means for allowing a feed-back control of the air-fuel ratio to be
carried out by the control means when the determination of the
activated condition of the oxygen sensor by one of the first and
second determination means is obtained.
3. An internal combustion engine according to claim 2, wherein said
sensor means is a lean sensor capable of producing an electric
signal having a level which is continuously varied in accordance
the air-fuel ratio.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for determining an
activated condition of an oxygen sensor.
2. Description of the Related Art
In an internal combustion engine for obtaining a desired air-fuel
ratio, an oxygen sensor, such as a lean sensor, is used for
obtaining an output signal which corresponds to the density of the
oxygen of the exhaust gas from the engine. The output signal from
the oxygen sensor controls a duration of a time of the opening of
the fuel injector, to obtain an air-fuel ratio of the internal
combustion engine set to a value larger than the stoichiometric
air-fuel ratio.
A known oxygen sensor includes a body of a solid electrolyte, such
as a stabilized zirconia, a first air permeable electrode on one
side of the solid electrolyte body and in contact with the exhaust
gas to be detected, and a second air permeable electrode on the
other side of the solid electrolyte body and in contact with the
reference gas, such as atmospheric air.
In this type of sensor, an application of a direct current electric
voltage within a predetermined range across the electrodes obtains
an electric current, called a limit electric current, which is
maintained at a predetermined value in accordance with a density of
the oxygen in the exhaust gas. Therefore, the detection of the
output electric current from the oxygen sensor enables a value of
the density of the exhaust gas to be obtained, and using this
value, it is possible to estimate the value at which the air-fuel
ratio for the internal combustion engine should be set.
Nevertheless, a low temperature of the solid electrolyte body
(i.e., element temperature) in this type of oxygen sensor causes
the sensor output value to be reduced while maintaining the same
air-fuel ratio within a range at which it is used. Namely, the
electric current value obtained corresponds to an oxygen density
value which is lower than the actual value of the oxygen density,
and as a result, a feed-back control of the air-fuel ratio in
accordance with the output signal from the sensor during a low
temperature state of the element causes the air-fuel ratio to be
incorrectly controlled to a value larger than the preset air-fuel
ratio.
One solution to such a problem is to provide a means of determining
if a state is reached wherein the oxygen sensor is "fully
activated", before executing the feed-back control of the air-fuel
ratio. A prior art method of obtaining such a determination of the
activated condition of the oxygen sensor uses the output level from
the oxygen sensor when the engine is under a fuel cut (F/C)
condition. This technique employs a principle such that the oxygen
density of the exhaust gas during the fuel cut operation is equal
to the oxygen density of the atmospheric air, and a determination
of whether or not the oxygen sensor is in an activated state is
obtained by determining if the output level from the oxygen sensor
corresponds to the oxygen density of the atmospheric air. (See
Japanese Unexamined Patent Publication No. 59-46350.) In this prior
art, a threshold level is provided and it is determined whether the
output level from the oxygen sensor has become higher than the
threshold level during the fuel cut condition. When the output
level from the oxygen sensor is higher than the threshold level, it
is determined that the element temperature is high enough to ensure
that the oxygen sensor is in an activated condition.
To improve the precision of the determination of the activated
state of the oxygen sensor, a technique has been proposed whereby
it is determined whether a condition, wherein the output level from
the oxygen sensor is higher than the threshold value during the
fuel cut condition, is continued. (See Japanese Unexamined Patent
Publication No. 60-21265.) In this prior art, a measurement of the
output of the oxygen sensor is repeatedly carried out at
predetermined intervals during the fuel cut condition, and a
determination of the activated state of the oxygen sensor is
obtained when a continuation of a state wherein the output level of
the oxygen sensor is higher than the threshold value for longer
than a predetermined interval is determined.
In this prior art, the reason for the determination of the
continuation of the state wherein the output level of the oxygen
sensor is higher than the threshold value for longer than the
predetermined interval, is to obtain a determination of the
activated condition of the oxygen sensor as quickly as possible
without losing the precision of the determination. The threshold
value for determining the activation condition is usually the
lowest possible value for obtaining a quick determination of the
activated condition of the element, and thus an incorrect
determination is apt to be made, since inevitably some fluctuation
of the output level from the oxygen sensor will occur, which causes
a situation to arise such that the output level from the oxygen is
higher than the threshold value even if the sensor is not actually
activated. Such an incorrect determination due to the fluctuation
of the output level is avoided by determining if the sensor output
level remains higher than the fixed value for a time longer than a
predetermined interval.
The above-mentioned prior art suffers from a drawback in that a
proper determination of the activated condition of the oxygen
sensor cannot be always obtained, for the following reason. Where
the engine is in a cruising state, e.g., when running on a freeway,
if a steady state condition continues for a relatively long time, a
fuel cut operation occurs only rarely. In this case, the prior art
method of determining the activated state of the oxygen sensor by
the detection of duration of the state where the sensor level is
higher than a predetermined level for longer than the predetermined
interval during the fuel cut operation, may sometimes cause the
air-fuel ratio feedback control to be stopped for a relatively long
time, and thus a precise control of the air-fuel ratio cannot be
obtained.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a device for
obtaining a quick and precise determination of a thermally
activated condition of the oxygen sensor, which increases the
occurrences of the air-fuel ratio feedback control operation, to
obtain a precise control of the air-fuel ratio to the desired
value.
According to the first aspect of the present invention, a device is
provided for determining an activated condition of an oxygen sensor
for an internal combustion engine where a fuel cut operation is
carried out under a predetermined condition of the engine, said
device comprising:
means for detecting a state where the fuel cut operation is carried
out;
a first comparing means for comparing the detected level of the
sensor means with a predetermined first value when the engine is
under a fuel cut condition;
a second comparing means for comparing the detected level of the
sensor means with a predetermined second value which is lower than
said first value when the engine is under a fuel cut condition;
a first determining means for determining an activated condition of
the oxygen sensor when the first comparing means determines that
the detected level once exceeds the first value, and;
a second determining means for determining an activated condition
of the oxygen sensor when the second comparing means determines
that the detected level consecutively exceeds the second value at
least twice.
According to the second aspect of the present invention, an
internal combustion engine is provided, comprising:
an engine body;
an intake line connected to the engine body for an introduction of
intake air into the engine body;
a means for introducing an amount of fuel into the intake line for
forming an air-fuel mixture;
an exhaust line connected to the engine body for a removal of a
resultant exhaust gas from the engine body;
a means arranged in the exhaust line and responsive to the oxygen
density of the oxygen in the exhaust gas for producing an electric
signal having a level which indicates the air-fuel ratio of the
combustible air-fuel mixture introduced into the engine;
a means, responsive to the signal from the sensor means, for
controlling the amount of the supplied fuel, to thereby control the
air-fuel ratio to a predetermined value;
a means, responsive to a predetermined condition of the engine, for
cutting the supply of fuel to the engine;
a means for cutting the supply of fuel to the engine from the fuel
supply means;
a first comparing means for comparing the detected level of the
sensor means with a predetermined first value when the engine is
under a fuel cut condition;
a second comparing means for comparing the detected level of the
sensor means with a predetermined second value which is lower than
said first value when the engine is under a fuel cut condition;
a first determining means for determining an activated condition of
the oxygen sensor when the first comparing means determines that
the detected level once exceeds the first value;
a second determining means for determining an activated condition
of the oxygen sensor when the second comparing means determines
that the detected level consecutively exceeds the second value at
least twice, and;
means for allowing the feed-back control of the air-fuel ratio to
be carried out by the control means when a determination of the
activated condition of the oxygen sensor by at least one of the
first and second determining means is obtained.
BRIEF DESCRIPTION OF ATTACHED DRAWING
FIG. 1 is a schematic view of the internal combustion engine
according to the present invention.
FIG. 2 is a schematic view of a control circuit shown in FIG.
1.
FIG. 3 shows a relationship between the air-fuel ratio and an
output level from the oxygen sensor.
FIGS. 4 to 7 are flow charts showing the operation of the control
circuit shown in FIG. 2.
DESCRIPTION OF A PREFERRED EMBODIMENT
FIG. 1 shows an internal combustion engine 2 according to the
present invention, which includes an intake pipe 4 in which a
throttle valve 8 is arranged. The throttle valve 8 is connected to
an accelerator pedal 6 by a link 7, and thus a degree of opening of
the throttle valve 8 is controlled in accordance with a depression
of the accelerator pedal 6. An air-flow meter 10 for detecting an
amount of air introduced into the engine is arranged in the intake
pipe 4 upstream of the throttle valve 8. A sensor 12 for detecting
the temperature of intake air introduced into the engine is
arranged in the intake pipe 4 between the throttle valve 8 and the
air-flow meter 10, and a throttle position sensor 14 is connected
to the throttle valve 8. The sensor 14 comprises a sensor for
detecting a continuously varied degree of opening of the throttle
valve 8, and an idling switch which is made ON when the throttle
valve 8 is in the idling position and is made OFF when the throttle
valve 8 is opened from the idling position.
An oxygen sensor 20 (i.e., lean sensor) is arranged in the exhaust
pipe 18 for detecting an air-fuel ratio of the combustible mixture
introduced into the engine, in response to an oxygen density of the
exhaust gas in the exhaust pipe 18. A catalytic converter 22 is
arranged in the exhaust pipe 18 downstream of the oxygen sensor
20.
Reference numeral 24 denotes a distributor having a distributor
shaft 24a connected to a crankshaft of the engine. The distributor
24 is connected to an ignitor 32, and a spark plug 34 is arranged
on a cylinder head and a high voltage electric current is supplied
to the spark plug 34 from the ignitor 32 via the distributor 24,
for carrying out an ignition of fuel.
A fuel injector 50 is arranged in the intake pipe 4 to generate a
flow of injected fuel directed into an intake port 3 of the
engine.
To detect various engine operating parameters, in addition to the
above-mentioned air flow meter 10, an intake air temperature sensor
12, a throttle position sensor 14 and the oxygen sensor 20, the
internal combustion engine 2 is provided with a first crank angle
sensor 26 facing the distributor shaft 24a and outputting a pulse
signal at every 30 degrees rotation of the crankshaft of the
engine, to calculate the engine rotational speed NE, a second crank
angle sensor 28 also facing the distributor shaft 24a and
outputting a pulse signal at every 720 degrees rotation of the
crank shaft, to determine a cylinder number, and a sensor 30
mounted on the cylinder block 5 body of the engine and in contact
with cooling water in a water jacket 7 of the engine, for detecting
the temperature of the cooling water for the engine 2.
These sensors are connected to an electronic control unit 40, which
is a microcomputer system, and signals from the sensors are input
to the unit 40.
As shown in FIG. 2, the electronic control unit 40 includes a
central processing unit (CPU) 40a, a read only memory (ROM) 40b for
storing various programs and data such as values of a threshold
level for determining an activated condition of the oxygen sensor
20, a random access memory (RAM) 40c as a means for temporarily
storing programs and data, and a back-up RAM 40d which is always
supplied with power by a battery (not shown) so that data is not
erased if a main power supply is cut. A bus 40e interconnects these
elements. The electronic control unit 40 is further provided with a
input interface 40f and an output interface 40g, and the air flow
meter 10, intake air temperature sensor 12, throttle position
sensor 14, oxygen sensor 20, first and second crank angle sensors
26 and 28, and engine cooling water temperature sensor 30 are
connected to the input interface 40f. Also, in addition to an
ignitor 32 and fuel injectors 50, an indicator 52 is connected to
the output interface 40g, to indicate whether the oxygen sensor 20
is activated.
The electronic control unit 40, which is responsive to various
engine operating conditions detected by the sensors, executes the
control of the amount of the fuel injected from the fuel injectors
50 and a control of the ignition timing by the ignitor 32.
Furthermore, the electronic control unit 40 monitors the output
level of the oxygen sensor 20, for determining an activated
condition of the oxygen sensor 20 according to the present
invention.
FIG. 3 shows the relationships between the air-fuel ratio of the
combustible mixture introduced into the engine and an output
electric current obtained by the oxygen sensor 20 arranged in the
exhaust pipe 18. A curve D shows a linear relationship between the
air-fuel ratio and the output current, obtained when the sensor 20
is completely activated. Curves A, B and C show the same
relationships obtained when the sensor element temperature is
660.degree. C., 640.degree. C., and 620.degree. C., respectively.
According to the present invention, two different values of
reference levels I.sub.1 and I.sub.2 are provided. When the output
level from the oxygen sensor 20 once exceeds the higher reference
value I.sub.1, it is determined that the sensor 20 is activated.
Contrary to this, when the output level of the sensor 20 exceeds
only the lower reference revel I.sub.2, the determination of the
activated condition of the oxygen sensor 20 is obtained when this
occurs twice in succession. Accordingly, a precise and quick
determination of the activated condition of the oxygen sensor 20
can be obtained.
The determination of the activated condition of the oxygen sensor
by any one of the first and the second reference values I.sub.1 and
I.sub.2 allows the determination to be made quickly and precisely.
Namely, the determination of the activated condition of the oxygen
sensor is obtained when the detected value of the electric current
from the oxygen sensor 20 under the fuel cut condition, where the
gas to be detected has the same air-fuel ratio as that of the
atmospheric air, is once higher than the higher reference value
I.sub.1 or is twice in succession higher than the lower reference
I.sub.2. As shown in FIG. 3, the higher reference value I.sub.1,
with respect to the air-fuel ratio during the fuel cut condition,
(A/F).sub.FCUT, corresponding to the oxygen density of the
atmospheric air, is between a value of the electric current on the
curve A and a value of the electric current on the curve B. The
lower reference value I.sub.2, with respect to the air-fuel ratio
during the fuel cut condition, (A/F).sub.FCUT, corresponding to the
oxygen density of the atmospheric air, is between a value of the
electric current on the curve B and a value of the electric current
on the curve C.
Now, an air-fuel ratio control operation of the electronic control
unit 40 will be described with reference to the flowcharts shown by
FIGS. 4 to 7. The fuel injection can be independently carried out
at desired timings in one cycle of the engine. FIG. 4 shows a fuel
injection routine for each of the injectors 50, which is executed
during one operation cycle of the engine. As is well known, this
timing is detected by the number of a counter (not shown), which is
incremented at every 30 degrees pulse signal output from the first
crank angle sensor 26 and cleared at every 720 degrees pulse signal
output from the second crank angle sensor 28. The routine in FIG. 4
is commenced at this timing, and at step 100 it is determined if
the flag FC is set. As will be described later, the flag FC is set
at (1) when the engine is under a fuel cut operation, and is reset
to (0) when the engine has finished the fuel cut operation. When it
is determined that the engine is under the fuel cut condition
(FC=1), the routine goes to step 102 and a zero value is moved to a
fuel injection amount .tau., whereby the fuel cut operation is
carried out.
When engine is not under a fuel cut operation (FC=0), the routine
goes from step 100 to step 104, where a basic fuel injection amount
.tau..sub.p is calculated by
where Q is an intake air amount detected by the air-flow meter, and
N is engine speed calculated as a time difference between adjacent
30 degrees pulses from the first crank angle sensor 26. This basic
fuel injection amount .tau..sub.p is used for obtaining a
theoretical air-fuel ratio at the detected Q and N.
At step 106, a final fuel injection amount .tau. is calculated
by
where FAF is a feedback correction factor, as will be described
later, KLEAN is lean correction factor as will be also described
later, and .alpha., .beta., and .gamma. are other correction
amounts or correction factors applied to the calculation of the
fuel injection amount, a detailed explanation of which is omitted
since they are not closely related to the invention.
At step 108, a fuel injection signal is sent from the output
portion 40g to the fuel injector 50, for carrying out a fuel
injection operation, and an amount of fuel calculated at step 106
is injected into the engine.
FIG. 5 shows a fuel cut condition determination routine, which is
executed at a predetermined interval. At step 110, it is determined
if the idle switch of the throttle position sensor 14 is ON, i.e.,
the throttle valve 8 is in the idling position. When it is
determined that the idle switch is OFF, the routine goes to step
112 and the fuel cut flag FC is cleared (0). When it is determined
that the idling switch is ON, the routine goes to step 114 and it
is determined whether the fuel cut flag FC is set to (1). When
FC=0, i.e., a fuel cut operation is not carried out during the
timing at which this routine is carried out in the preceding
routine, the routine goes to step 116 and it is determined whether
the engine speed NE is larger than a predetermined value NE.sub.1.
When NE>NE.sub.1, this means that the engine has been
decelerated from a condition wherein the engine speed was higher
than NE.sub.1. The routine then goes to step 118 and the fuel cut
flag FC is set to (1 ), and therefore, a fuel cut operation is
carried out (step 102 in FIG. 4).
At the following timing for executing the routine of FIG. 5, the
routine goes from step 114 to step 120 and it is determined whether
the engine speed NE is larger than a predetermined value NE.sub.2,
which is smaller than NE.sub.1. When it is determined that
NE>NE.sub.2, the routine goes to step 118 and the fuel cut
operation is maintained. When the engine speed dropped below the
level of NE.sub.2, the routine goes from step 120 to the step 112,
and the fuel cut operation is cancelled.
FIG. 6 shows a feedback correction factor calculation routine which
is executed at predetermined intervals such as 4 milliseconds. At
step 120, it is determined if a flag F.sub.1 is set. As will be
explained later, this flag is set at (1) when the oxygen sensor 20
is in an activated condition, and is reset to (0) when the oxygen
sensor 20 is not activated. When the oxygen sensor 20 is not
activated (F.sub.1 =0), the routine goes to step 122, and the steps
following step 122 to execute the open loop control. During the
open loop control, at step 122, a value of 1.0 is moved to the lean
correction factor KLEAN, and the flow goes to step 124 where a
value of 1.0 is moved to the feedback correction factor FAF.
Because KLEAN=1.0, as a result of the execution of the open loop
control, the air-fuel ratio is basically determined by the basic
fuel amount .tau..sub.p calculated at step 106 in FIG. 4.
When it is determined that the oxygen sensor 20 is activated at
step 120 (F.sub.1 =1), the routine goes to step 126 and it is
determined if other feedback conditions are satisfied. The feedback
control is stopped when, for example, the engine is under an
acceleration condition. When the feed back condition is not
satisfied, the routine goes from step 126 to step 122, and the
steps following step 122, to execute the open loop control of the
air-fuel ratio. When it is determined that the feedback conditions
are satisfied, the routine goes to step 128 and a lean correction
factor KLEAN is calculated. The lean correction factor may have
values lower than 1.0, and as shown in the step 106 in FIG. 4, is
multiplied with the basic fuel amount .tau..sub.p, to obtain a lean
air fuel mixture. As is well known, a map of data of the values of
the lean correction factor with respect to combinations of the
engine speed NE and engine load, such as a ratio of the intake air
amount Q to the engine speed NE is provided. A well known map
interpolation calculation of a value of the lean correction factor
KLEAN is carried out by using the detected values of NE and Q/NE.
At step 130, the value of the electric current Ox from the oxygen
sensor 20 is input, and at step 132, a corrected electric current
IR, which indicates the air-fuel ratio of the combustible mixture,
is calculated from the detected electric current by the oxygen
sensor 20. The two-dimensional map provided has the values of the
IR with respect to the detected electric current, and at step 134,
a reference value IR' of the corrected electric current, i.e., a
target value of the air-fuel ratio, is calculated from the lean
correction factor KLEAN calculated at step 128.
The processes of steps 136 to 148 generally show a air-fuel ratio
feedback routine. At step 136, it is determined if the IR
(corresponding to the detected air-fuel ratio) is larger than the
IR' (corresponding to the target air-fuel ratio). When it is
determined that IR>IR', i.e., the detected air-fuel ratio is
higher than the target air-fuel ratio, the routine goes to step 138
and it is determined if the first determination of IR>IR' at
step 136 is obtained. If the result of the determination at the
step 138 is yes, the routine goes to step 140 and the feedback
correction factor FAF is incremented for a value of A, i.e., a skip
operation is carried out. When it is determined that the result of
IR>IR' at step 136 is not the first result, the routine goes to
step 142 and the feedback correction factor FAF is incremented for
a value of a, which is smaller than A. When it is determined that
IR.ltoreq.IR', i.e., the detected air-fuel ratio is equal to or
smaller than the target air-fuel ratio, the routine goes to step
144 and it is determined if the first determination of
IR.ltoreq.IR' at step 136 is obtained. If the result of the
determination at the step 144 is yes, the routine goes to step 146
and the feedback correction factor FAF is decremented for a value
of B, i.e., a skip operation is carried out. When it is determined
that the result of IR.ltoreq.IR' at step 136 is not the first
result, the routine goes to step 148 and the feedback correction
factor FAF is decremented for a value of b, which is smaller than
B.
FIG. 7 shows a routine for determining an activated state of the
oxygen sensor 20, and is carried out at a predetermined interval
of, for example, 2, 18 or 25 seconds. At step 160 it is determined
if the fuel cut flag FC is set. When the FC=0, i.e., the fuel cut
operation is not carried out, the routine following step 162 is
by-passed. When it is determined that the FC=1, i.e., the fuel cut
operation is being carried out, the routine goes to step 162 and it
is determined if the corrected output current IR from the oxygen
sensor 20 is larger than a predetermined second or lower level
I.sub.2. As will be described later, when two consecutive
determinations of IR>I.sub.2 are obtained, a determination of an
activated condition of the oxygen sensor 20 is obtained even if the
output current IR does not exceed the higher or first level
I.sub.1. When it is determined that the IR is lower than the lower
level, i.e., the oxygen sensor 20 is not activated, the routine
goes to step 164 and the flags F.sub.1, F.sub.2 are cleared. When
the flag F.sub.1 is cleared (F.sub.1 =0), the routine at step 120
in FIG. 6 flows to steps 122 and 124, and therefore, the feedback
control of the air-fuel ratio is not carried out, and thus an open
control of the air-fuel ratio is carried out to obtain the
theoretical air-fuel ratio. When it is determined that
IR>I.sub.2 at step 162 in FIG. 7, the routine goes to step 166
and it is determined whether the corrected output current IR from
the oxygen sensor 20 is larger than a predetermined first or higher
level I.sub.1. When it is determined that the IR is higher than the
first or higher level I.sub.1, i.e., the oxygen sensor 20 is
activated, the routine goes to step 168 and the flags F.sub.1 is
set. When the flag F.sub.1 is set (F.sub.1 =1), the routine at step
120 in FIG. 6 flows to steps 126 and 128, and therefore, the
feedback control of the air-fuel ratio is carried out to obtain a
lean air-fuel ratio in accordance with the engine operating
condition. When it is determined that the IR is lower than the
first or higher level I.sub.1 but higher than the second or lower
level I.sub.2, the routine goes to step 170 and it is determined if
the flag F.sub.2 is set. When a result F.sub.2 =0 is obtained at
step 170, i.e., when the first determination of IR>I.sub.2 at
the step 162 is obtained, the routine goes to step 172 and the flag
F.sub.2 is set and the flag F.sub.1 is held in the reset state
(F.sub.1 =) so that the open loop control of the air-fuel ratio is
maintained. At the following timing for executing the routine in
FIG. 7, if the condition I.sub.2 <IR<I.sub.1 is maintained,
the routine goes from step 170 to step 168 and the flag F.sub.1 is
set, whereby a determination of the activated condition of the
oxygen sensor 20 is obtained and the feedback control is
commenced.
As is clear from the above, when a result of a determination of the
IR between I.sub.2 and I.sub.1 is obtained, a determination is made
at step 170 if the temporary flag F.sub.2 is set at the execution
of this routine at the preceding timing. If the result of the
determination at step 170 is no, then the temporary flag is set,
and thus the result of the determination of the step 170 becomes
yes and the flag F.sub.1 is set if the result of the value of the
IR between I.sub.2 and I.sub.1 is again obtained at the execution
of the step 166 during the following timing of the execution of the
routine. This means that the temporary flag F.sub.2 is set when two
consecutive determinations that the IR is between I.sub.2 and
I.sub.1 are made, and if such two consecutive determinations are
obtained, it is considered that the oxygen sensor is activated, and
thus the air-fuel ratio feed back control is commenced. As a
result, a precise and rapid determination of the activated state of
the oxygen sensor can be obtained.
When the engine is operating under a continuous steady state for a
relatively long time, such as running on a free way, a fuel cut
operation rarely occurs, and thus the higher first threshold value
I.sub.1, which is determined only once, is effective for obtaining
a quick determination of the activated condition of the oxygen
sensor 20. The lower second threshold value I.sub.2, which is used
twice consecutively, is effective for a positive and quick
determination of the activated condition of the oxygen sensor when
the output value of the oxygen sensor as activated is relatively
small, as when a cold engine is started.
Although an embodiment of the present invention is described above
with reference to attached drawings, it is obvious that many
modification and changes can be made by those skilled in this art
without departing from the scope and spirit of the present
invention.
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